Production of free-machining stainless steel



' Patented Nov. 11, 1941 UNITED STATES PATENT OFFICE PRODI JCTION 0F FREE-MACHINING STAIN- LESS STEEL Vincent '1. Malcolm, Indian Orchard, Mass., as-

signor to The Chapman Valve Manufacturing 00., Indian Orchard, Mass., a corporation of Massachusetts No Drawing. Application June 22, 1940, Serial No. 341,961

2 Claims.

herent in steels of the same type which have a relatively high sulphur content but none of their disadvantages.

It has for some time been the practice to make chromium steels for use in the manufacture of such machined parts as valve stems, other valve parts, etc., by means of such machines as socalled automatic screw machines which operate at high speeds, but success has only been had, as far as efliciency, speed,- and economy are concerned, when these steels have had a relatively high sulphur content, say around .25 to 50%. However, the oil, chemical, and other industries which employ machined steel parts under such conditions that they are subjected to the action .of various gases and acids at high temperatures have for some time objected to the corrosiveness and high degre of embrittlement in such steels.

That is to say, while 11-14 chromiumsteels having this relatively high sulphur content are capable of rapid and efflcient machining, the sulphurcontent at the same time has been more or less of a contaminating element wherefore such steels are not satisfactory for use in these industries. I l 1 Other chromium steels containing ,relatively less sulphur have, of course,- been produced but they are not readily adapted for machining in automatics. That is to say, 11-14 chromium steels 3.)

containing a relatively low amount of sulphur have been made but they have never been used with success inautomatic machines which operate at high speeds since the cutting speed of such low-sulphur chromium steels is about 30 feet per minute as contrasted with a cutting speed of around 100 feet per minute of a chromium steel containing a larger amount of sulphur. v

Further, the outside threading and'flnishing of a chromium steel having a low sulphur content on an automaticl'high-speed machine isarough and of poor quality and tapping thereon is prac- -tically, impossible.' That is, suchsteels have v to be removed from theautomatic inachin'es'and tapped and threaded by- 'more or less of ahand method.

- It is, accordlnglyp the principal object of this invention 'to provide a 11-14%' chromium steel which does not' have the corrosiveor qbjectionably brittle qualities of' such steels containingthe 55 cessive grain size formation and then it is quickly brought to about 1580" F. where it; is rapidly relatively high amount of sulphur but which has their fine machining qualities.

chromium steel having a low sulphur content and give to it the machining properties of a similar-'- steel having a. higher sulphur content'while eliminating the contaminating influence thereof.

The formula of a common 11-14% chromium steel having a relatively high sulphur content and which has met with'success as far' as its machinabil ity is concerned, is'as follows: Carbon per cent maximum .15

' Manganese do .50 Silicon do .50 Phosphorus do .04 Nickel do .50 Chromium do.. 11.5-13.0 Sulphur, approximatelyrz nnper cent .25 to .50 As stated, however, such steels are not adapted for 'use in oil refineries, for examplawhere the machined valve stems or the like must withstand the action of acids and gases at elevated temperatures. According to this invention, I employ a steel having the same approximate formula set forth above except that the sulphur content of the steel I work preferably has a maximum sulphur content of .05%.

The preferred process of heat treating this type of steel will now be described.

The piece or pieces are first heated slowly in e a suitable furnace to a temperature below the critical transformation point of the steel preferabl'y to about 1450". F. The steel is soaked or held at this temperature until the heat penetration is substantially complete, this usually taking about one and one-halfhours for each inch of thickness.

After this the pieces are heated rapidly to a higher temperature which, nevertheless, is still below a temperature at which the carbides have not gone. completely into solution and preferably between 1550 F. and 1625 F., from which temperature the steel is then cooled rapidly:

either in air or oil.

1 .It will be noted that all heating is at a temperature below the complete carbidesolution point. The steel is brought to a uniformheat condition throughout at a .temperature of approximately 1450 F; without the danger of excooledfor quenched in air 'or oil. The above process has the advantages of providingreally machinable 11-14 chromium: steels,

In other words,- by the method of this invention, 1 take a 11-14% thereof, it is not desired as contrasted with stainless steels hitherto made which are too hard or tough formy purposes. Further, according to thi invention more uniform results can be obtained, especially lnas-' much as a whole batch of pieces may be quenched simultaneously with uniformity.

It will also be obvious that any subsequent heat-treating steps are not'always necessary and that my process, being relatively simple and easy to perform, requires less'care in its operation. Thus it is more economical.

Manifestly the specific details of construction may be considerably varied from those herein shown and described without'involving any departure from the principle of the invention or sacrificing any of the advantages inherent therein. While I have described my invention in this specification in great detail and particularly with respect to the present preferred form to be limited thereto since many changes andv modifications may be made therein without departing from the spirit and scope of the invention.

The invention may be embodied in other specific forms without departing from the essential characteristics thereof. Hence, the present embodiments are therefore to be considered in all respects merely as being illustrative and not as being-restrictive, the scope of the invention being indicated by the appended claims 'perature below its critical rather than by the foregoing description, and all modifications and variations as fall within the meaning and purview and range of equivalency or the appended claims are therefore intended to be embraced therein.

'What it isdesired to claim and secure b Letters Patent of the United States is:

1. The process of treating 11-14% chromium f steel so as to render the sam free-machining which consistsin, heating the steel to a temtemperature point and around 1450" F., then stepping up the heat to a temperature around 1580" quenching the steel rapidly, whereby the structure' produced is partially martensitic together with carbides and the steel is in a semi-hardened condition. 2. The process of treating 1114% chromium steel so as to render the sam'e free-machining which consists-in, heating the steel to a temperature below its critical transformation point and around 1450 F'., then stepping up the heat to a temperature plete carbide solution point and around 1580 F., and finally quenching the steel rapidly, whereby the structure produced will be partially mar tensitic together with carbides and the steel is in a semi-hardened condition.

vmcEN'i' 'r. MALCOLM.

F., and finally v which is still below the com- 

